The present invention relates to a perfected electromagnetic metering valve with a ball shutter for a fuel injector, and in particular for internal combustion engines.
The metering valves of fuel injectors normally comprise a control chamber with a discharge conduit, which is normally closed by a very hard ball. In known metering valves, the ball is normally kept in the closed position inside a seat of the discharge conduit by a spring, and, when a control electromagnet is energized, the armature frees the ball from the spring to open the conduit. On account of the high pressure of the fuel inside the control chamber, a very strong spring is required to close the valve.
In known valves, the armature is connected to a stem, which is pressed by the spring directly on to the shutter, so that contact between the stem and the ball is substantially point-to-point. This results in severe pressure between the ball and stem, and, as the ball is made of harder material than the stem, eventually in deformation of or wear marks on the stem, which eventually affect the travel of the ball.
As is known, the travel of the ball, and hence of the armature of the electromagnet, is one of the main parameters determining the amount of fuel injected into the engine. That is, for a given length of time the electromagnet is energized, and for a given supply pressure, an increase in the travel of the ball corresponds to an increase in the opening and closing time of the valve, and hence an increase in the amount of fuel injected.
Moreover, inevitable tolerances in the manufacture of the transmission mechanism between the armature and the ball make it extremely difficult to perfectly align the armature, stem and ball seat axes, so that known valves present the problem of activating the ball in a perfectly axial direction with respect to the seat, i.e. eliminating the transverse components generated by the action of the stem on the ball. Also, to ensure effective sealing of the valve, the active surface of the armature must be perfectly parallel to the seating surface of the ball.
Metering valves of the above type are known in which the valve seat is conical to avoid contact of the ball with a sharp edge; and a further metering valve has been proposed in which the stem comprises a spherical cavity enclosing the ball. Such a cavity, however, allows of no lateral movement of the ball, so that, in the event of misaligned activation of the ball, due to inevitable assembly tolerances of the valve components, the ball is incapable of centering itself automatically.